On the effect of pore-space properties and water saturation on explosive spalling of fire-loaded concrete

In this paper, the influence of concrete properties such as permeability and environmental conditions (water content) on the spalling behavior of concrete subjected to fire loading is investigated. For this purpose, a special fire-test setup is presented, allowing an improved reproduction of temperature histories observed, e.g., in tunnel fires. Moreover, the developed test setup enables a continuous monitoring of spalling, giving access to the spalling history and the final level of damage of concrete specimens. The obtained results, considering different water/cement-ratios and saturation degrees, are related to the required initial tensile strength determined by means of a numerical assessment tool. Finally, correlations between the identified parameters governing fire-spalling were established and allowed an evaluation of the spalling risk of concrete mixtures. These correlations revealed that – in contrast to requirements given in national and international standards – the combination of water content and permeability may serve as proper key and design parameter, defining the risk of spalling

Thermo-hydro-chemical couplings considered in safety assessment of shallow tunnels subjected to fire load

Fire loading of concrete tunnel linings is characterized by various physical, chemical, and mechanical processes, resulting in spalling of near-surface concrete layers and degradation of strength and stiffness of the remaining tunnel lining. In this paper, the governing transport processes taking place in concrete at elevated temperatures are considered within a recently published fire-safety assessment tool [Savov K, Lackner R, Mang HA. Stability assessment of shallow tunnels subjected to fire load. Fire Safety J 2005; 40: 745-763] for underground structures. In contrast to consideration of heat transport only [Savov et al.], a coupled thermo-hydro-chemical analysis, simulating the heat and mass transport in concrete under fire loading, is performed, giving access to more realistic temperature distributions as well as gas-pressure distributions within the tunnel lining. These data serve as input for the structural safety assessment tool considering, in addition to the temperature dependence of mechanical properties, the effect of the gas pressure on the strength properties of the heated lining concrete. The combination of the two analysis tools (coupled analysis of governing transport processes and structural safety assessment) is illustrated by the fire-safety assessment of a cross-section of the Lainzer tunnel (Austria) characterized by low overburden (shallow tunnel)